Diesel exhaust system including NOx-trap

ABSTRACT

An exhaust system for a diesel engine, which engine being operable in a plurality of modes including an idling mode, wherein the engine emits a relatively cool exhaust gas, and a running mode, wherein the engine emits a relatively hot exhaust gas, includes a solid NOx absorbent, whereby NOx is absorbed by the NOx absorbent during the idling mode and is desorbed and passed to atmosphere during the running mode and/or during acceleration from idling to running as the temperature of the exhaust gas increases, wherein during all modes the engine runs lean and the exhaust gas composition is lambda&gt;1.

This application is the U.S. national phase application of PCTInternational Application No. PCT/GB01/02483.

FIELD OF THE INVENTION

The present invention relates to an exhaust system for a diesel engine,and in particular to an exhaust system including a NOx-trap or NOxabsorbent.

BACKGROUND OF THE INVENTION

Diesel engines produce an exhaust gas including inter alia carbonmonoxide (CO), hydrocarbons (HC), soot and nitrogen oxides (NOx), whichNOx including nitrogen oxide (NO) and nitrogen dioxide (NO₂). NO₂ istoxic and can cause headaches, dizziness and nausea in low doses. Italso has an objectionable smell. CO, NOx, soot and HC are legislated sothat the levels emitted from a diesel exhaust system to atmosphere mustmeet prescribed limits. Thus a diesel oxidation catalyst catalyses theoxidative removal of HC, CO and soot as does Johnson Matthey'sContinuously Regenerating Trap (CRT™) (see EP-A-0341832). Many vehicles,including buses and trains, are powered by heavy-duty diesel powerplants (as defined by the relevant European, US Federal or Californianlegislation), all of which met the legislation existing at the time oftheir production.

EP-A-0560991 and EP-A-0758713 describe processes for removing NOx fromthe exhaust gas from diesel engines by absorbing it on a solid absorbentwhile the gas is lean (i.e. lambda>1) as in normal lean-bum engineoperation and regenerating the absorbent by intermittently adjusting thegas to a stoichiometric or rich composition. The intermittent adjustmentrequires engine inlet modification and/or reductant injection, andtherefore is unattractive for existing and immediately-availablevehicles because of the expense and technical complexity.

The average speed of vehicles in towns and city centres is relativelylow. For example, we understand that the average speed in centralLondon, U.K. is about 4 mph. The levels of pollutants, including NO₂, incity centre locations can be relatively high, and in cities such asManhattan in New York, “canyons” are formed between tall buildings andthis can prevent polluted air from moving, mixing and becoming dilutedby “fresh” air from outside these canyons.

Whilst vehicles powered by diesel power plants used in e.g. city centresmeet legislative requirements for NOx, it would be desirable to gobeyond the relevant legislation in certain situations in order to reducethe exposure of, e.g. passengers boarding or alighting from a bus, toNO₂ and generally to reduce the level of NOx in city centres forenvironmental reasons.

SUMMARY OF THE INVENTION

We have now found that it is possible to store NOx exhausted from adiesel engine during low load, such idling in city-centre traffic, andto exhaust the stored NOx when traffic conditions permit higher loads onthe engine, such as during acceleration and/or at faster, cruisingspeeds.

According to one aspect the invention provides an exhaust system for adiesel engine, which engine being operable in a plurality of modesincluding an idling mode, wherein the engine emits a relatively coolexhaust gas, and a running mode, wherein the engine emits a relativelyhot exhaust gas, the exhaust system comprising a solid NOx absorbent,whereby NOx is absorbed by the NOx absorbent during said idling mode andis desorbed and passed to atmosphere during said running mode and/orduring acceleration from idling to running as the temperature of theexhaust gas increases, wherein during all modes the engine runs lean andthe exhaust gas composition is lambda>1.

This discovery has particular application in urban driving where, forexample, an exhaust system for a bus powered by a heavy-duty dieselengine can store NOx when the vehicle is idling e.g. picking up andsetting down passengers or crawling through city centre traffic, andexpel the stored NOx when the bus is able to move at faster speeds e.g.between bus stops or beyond the city centre. Thus, the invention reducesor meets the problem of exposing, e.g. bus passengers or city-centrepedestrians, to NO₂ exhausted from the vehicle's engine and cangenerally improve the air quality in city centres. Depending on thetemperature required to desorb the adsorbed NOx, NO₂ can be expelledbeyond the city centre where there are less people and it can be mixedand diluted with “fresh” air, or travelling between bus stops so thatthe offensive odour is less concentrated. Similar advantages are derivedby use of the invention in trains (passengers at stations are exposed toreduced NOx levels) and generally in vehicles powered by light- andheavy-duty diesel power plants used for urban driving.

DETAILED DESCRIPTION OF THE INVENTION

The diesel engine can be the motive power for a vehicle, especially oneoperated on a stop/start schedule, such as a taxi delivery van, omnibus,water-bus or passenger train. In idling mode, while passengers areboarding or in stationary or slow-moving traffic, odourless NO, withlittle NO₂, is emitted and undergoes very little oxidation to NO₂ owingto high dilution and very slow reaction. By suitable choice of absorbercomposition and expulsion temperature, the rate of desorption can beslow enough to occupy a substantial part of the vehicle moving time, sothat the expelled NOx (having a low NO₂:NO ratio at desorptiontemperature) becomes well diluted. The engine is of course to be usedonly where pollution regulations permit. The engine may needmodification to decrease its normal NOx output, but less than if theinvention were not used. Desirably, the engine is fuelled withlow-sulfur fuel, i.e. having less than 50, for example under 10, ppm ofsulfur, by weight as elemental S.

The exhaust system is used in association with a diesel engine, whetherlight- or heavy-duty. In preferred embodiments the exhaust systemcomprises, in order from upstream to downstream as appropriate: a NOxabsorber; an oxidation catalyst and NOx absorber; a soot filter and NOxabsorber; or an oxidation catalyst, soot filter and NOx absorber.

The most preferred embodiment is the above described combination ofoxidation catalyst, soot filter and NOx absorber. Such a combinationeffects oxidation of NO to NO₂, whereafter the NO₂ combusts sootcollected on the filter. Since some of the NO₂ is thereby reduced to NO,the resulting gas is especially suitable for a NOx absorber sized toabsorb mainly or only the NO₂ component Certain embodiments of thiscombination are commercially available as Johnson Matthey's CRT™ and aredescribed in U.S. Pat. No. 4,902,487 and EP-A-0341832, the teaching ofwhich are incorporated herein by reference.

The invention is especially beneficial when the exhaust system includesan oxidation catalyst, for example a 2-way catalyst (one that oxidisesCO and HC), since such a catalyst can convert NO to NO₂. Thus the levelof tailpipe NO₂ is increased. As mentioned above, an aspect of thepresent invention is to reduce or prevent NOx and especially NO₂ frombeing exhausted to atmosphere during city centre driving.

The nature of the absorbent can be: (a) compounds, preferably an oxide,of alkali metals, alkaline earth metals, rare earth metals andtransition metals capable of forming nitrates and/or nitrites ofadequate stability in absorbing conditions and of evolving nitrogenoxides and/or nitrogen in regenerating conditions; or (b) adsorptivematerials such as zeolites, carbons and high surface-area oxides, ormixtures of any two or more thereof.

In one embodiment, the absorbent can be catalysed, preferably with oneor more platinum group metal. Where an oxidation catalyst is present,this can facilitate oxidation of NO to NO₂ so that the NO₂ is stored asthe nitrate in the NOx absorbent. Without wishing to be bound by theory,we understand that when the exhaust temperature rises, the nitratebecomes thermally unstable releasing the NOx as NO₂. We have observed,however, that where an appropriate combination of absorbent materialsare used, the primary NOx component released can be NO and not NO₂ (withresidual oxygen).

Compounds (a) may be present (before NOx absorption) as mixtures and/orcomposite oxides, e.g. of alkaline earth metal and copper such asBa—Cu—O or MnO₂—BaCuO₂, possibly with added Ce oxide, or Y—Ba—Cu—O andY—Sr—Co—O. (For simplicity the oxides are referred to, but in situhydroxides, carbonates and nitrates are present, depending on thetemperature and gas composition). Whichever compounds are used, theremay be present also one or more catalytic agents, such as preciousmetals, especially platinum group metals (PGMs) such as platinum orpalladium, for oxidising NO to NO₂. However, it is a feature of theinvention that the absorbent need not include a reduction catalyst, suchas the PGM rhodium, since the composition of the exhaust gas accordingto the invention is always at lambda>1. Accordingly, because the gascomposition includes net oxidising species, it is more difficult toreduce NOx to N₂.

As is known to the skilled person, the exhaust gas temperatures ofheavy- and light-duty diesel engines are different. In particular, theexhaust gas temperature of a heavy-duty diesel engine in idling andrunning modes is higher than in a light-duty diesel engine. Accordingly,in preferred embodiments of the invention, the nature of the absorbentmaterial can be chosen better to match the exhaust gas temperaturesencountered in its application in exhaust systems for either heavy- orlight-duty diesel engines. More strongly basic oxides e.g. BaO arepreferred absorbent materials for heavy-duty applications. Particularlypreferred is a barium absorbent including a catalytically effectiveamount of a PGM e.g. platinum. Weaker basic oxides e.g. CuO or AgO arepreferred in light-duty applications. A mixture of copper oxide (200-600g/ft³) and palladium (50-200 g/ft³) has been found to be especiallyeffective.

Using the preferred absorbent for heavy-duty diesel, net NOx absorptionoccurs typically at up to 280° C.; it should take place at any lowertemperature at which the engine is producing NOx and is typicallyeffective at over 220° C. Net desorption of NOx from the absorbentoccurs at from 350-500° C. Using the preferred absorbent for light-dutydiesel applications, net absorption occurs typically at from 140-230° C.and net desorption at from 250-300° C. The transition from absorption todesorption in many systems according to the invention can be effectedsimply by the increase in exhaust temperature due to the change fromidling to running. However, a control system as described below can befitted as a safeguard.

Generally any idling period will in practice include a terminaltemperature rise as the transition to a running period. Desirably, thetemperature of the gas entering the absorber can be appropriatelycontrolled; if the temperature of the gas entering the absorber is toohigh, a simple cooling means, such as a finned connector can be insertedupstream of the absorber. If the temperature of the gas entering theabsorber is too low, a connector containing an electric heater can beinserted upstream of the absorber. Alternatively the temperature can beincreased by adjusting the engine inlet and/or by injecting fueldownstream of the engine, to provide an exotherm over the oxidationcatalyst Such adjustment and/or injection is preferably used only at thetransition from idling to running mode. The fuel thus introduced doesnot bring the gas composition outside the required lean range.

The absorber composition can be chosen to absorb preferentially NO₂, sothat the NO₂:NO ratio in the tailpipe gas in the idling mode isrelatively low, preferably at a level less than causes objectionableodour. In light-duty diesel application this can range from 5:1 to0.1:1, whereas in heavy-duty applications the ratio can be from 3:1 to0.001:1. A useful effect of low absorption of NO is that the volume ofthe absorbent is less than would be needed for both NO and NO₂, thusfacilitating retro-fit of a NOx trap in the confined space available ona vehicle.

In a preferred embodiment, the choice of absorbent depends on thetemperature available in the running mode. Whichever is used, thetransition from absorption corresponds to a change in the position ofequilibrium of the reaction:2NO+O₂→2NO₂,the reverse reaction being relatively favoured at desorptiontemperatures, so that the NO₂ content of the expelled gas is relativelylow.

According to a preferred embodiment, the invention provides a retro-fit(‘bolt-on’) NOx absorber with (if not already present) oxidationcatalyst or soot filter or both, and also with, if necessary,temperature adjustment means and/or control gear, for application to theengine exhaust pipe or to the outlet of an existing oxidation catalystand/or soot filter. It provides also combinations of the NOx absorberwith exhaust pipe parts, mufflers and muffler parts, catalysts and sootremovers, to be fitted to the engine/exhaust system in course ofmodification or maintenance.

The absorbent is suitably supported on a ceramic or metal honeycombsubstrate, the ceramic comprising one or more of alumina, silica,titania, cordierite, zirconia, silicon carbide or other, generallyoxidic, material. The substrate carries a washcoat and, in one or morelayers thereon, is the active absorptive material. The honeycomb hastypically 50-600 cells per square inch (cpsi), optionally more, e.g. upto 1200 cpsi or more if composed structurally of metal. Generally therange 100-900 cpsi is preferred. The absorbent or part thereof can be onthe outlet side of the filter, described below.

Where present, the oxidation catalyst, the active material comprisesgenerally a platinum group metal (PGM), especially platinum and/orpalladium, optionally with other PGMS, e.g. rhodium, and other catalyticor promoting components. The exact compositions and structure of theoxidation catalyst may be varied according to the requirements of itsapplication. A low temperature light-off formulation is generallypreferred. Usually such catalyst is supported on a structure of the sametype as described above for the absorbent Suitable PGM loadings are inthe range 20-200 g/ft³ on a honeycomb having 400 cpsi, withcorresponding loadings at other cell densities. The oxidation catalystmay be designed and/or operated to provide oxidation of gaseoushydrocarbons and CO, and/or of soot-carried hydrocarbons, but primarilyto oxidise NO to NO₂.

Where present, the soot filter is preferably capable of collecting sootwithout causing excessive back-pressure in the exhaust system. Ingeneral, ceramic, sintered metal or woven or non-woven wire filters areusable, and wall-flow honeycomb structures are particularly preferred.The structural material of the filter is preferably a porous ceramicoxide, silicon carbide or sintered metal. The filter can be catalysed,e.g. it may include an alumina coating and/or a base metal catalyst suchas La/Cs/V₂O₅. The soot is generally carbon containing soluble organicfractions (SOF) and/or volatile organic fractions (VOF) and/or heavyhydrocarbons. Combustion of soot produces carbon oxides and H₂O. Thesystem is advantageously applied to a heavy-duty engine, e.g. over 4litres, since the exhaust of such an engine can readily attain thepreferred temperatures as described herein. The filter in such a systemmay have sufficient capacity to operate without accumulation requiring agas bypass.

Advantageously, the system may further comprise sensors, indicators,computers and actuators, effective to maintain operation within desiredconditions. Preferably a means for controlling regeneration of the NOxabsorbent includes a computer which can be part of the engine managementunit if desired. Control of the system can be regulated with open orclosed feedback using information gathered from the sensors, indicators,etc.

Preferably the means for controlling the regeneration of the absorberperforms one or more of the following illustrative techniques:

(a) ultimate detection of NOx leakage from or “slip” past the NOxabsorber;

(b) regeneration responsive to prediction based on input of data ontimes in idling and running modes and rate of change from idling torunning and back; and

(c) allowance for gas composition variations, for example non-steadyconditions such as incomplete warm-up or weather.

The control means may include sensors for at least one of: fuelcomposition; air/fuel ratio; exhaust gas composition (includingtail-pipe NO₂) and temperature at one or more points along the exhaustsystem; and pressure drop, especially over the filter, where present. Itmay include also indicator means informing the engine operator, computermeans effective to evaluate the data from the sensor(s), and controllinkages effective to adjust the engine to desired operating conditionstaking account of e.g. start-up, varying load and chance fluctuations.

In addition, the system may include routine expedients, for exampleexhaust gas recirculation (E.G.R); and means such as cooling, orelectric heating, to adjust the temperature of the gas to a levelpreferred for nearer optimum operation of downstream components.

According to a further aspect, the invention provides a process fortreating diesel exhaust gas from an engine operated in a plurality ofmodes including an idling mode, wherein the engine emits a relativelycool exhaust gas, and a running mode, wherein the engine emits arelatively hot exhaust gas, which process comprising the steps ofabsorbing NOx on a regenerable NOx absorbent during said idling mode anddesorbing the NOx and passing it to atmosphere by intermittentlyincreasing the temperature of the exhaust gas, wherein during all modesthe engine runs lean and the exhaust gas composition is lambda>1.

According to a preferred embodiment the process, further comprises thesteps, before absorbing the NOx, of catalysing oxidation of NO to NO₂;collecting soot on a filter; and combusting said soot by reaction withsaid NO₂.

Most preferably, the process according to the invention is performed onan exhaust gas which is the product of combustion of a fuel containingless than 50 ppm w/w of sulphur.

Whereas in the prior processes referenced above the regeneration phaseusing rich gas is a small fraction of engine running time, in operatingthe exhaust system according to the invention the desorption phase willgenerally be long and can spread NOx desorption over a time and distancesufficient to avoid offence. Typically the ratio of running time toidling time is in the range 0.1 to 100.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more fully understood, reference willnow be made to the following illustrative Examples and to theaccompanying drawings, wherein:

FIG. 1 is a schematic illustration of a diesel exhaust systemrepresenting a preferred embodiment according to the invention;

FIG. 2 is a graph showing the variation with time of the composition ofa diesel exhaust gas from a diesel engine including an exhaust systemaccording to the invention; and

FIGS. 3 and 4 are graphs showing the variation with time of the NOxcomponents of a diesel exhaust gas from a diesel engine including anexhaust system according to the invention.

The system 8 comprises a “can” 10, connected at 12 to a conduit carryingthe exhaust from a diesel engine (not shown) fuelled with diesel fuel ofunder 50 ppm sulphur content, and having no exhaust treatment componentsupstream of the illustrated system. At the inlet end of can 10 iscatalyst 14, which is a low temperature light-off oxidation catalystsupported on a 400 cells/in² ceramic honeycomb monolith. Catalyst 14 isan oxidation catalyst designed to be capable of meeting emissionregulations in relation to CO and HC for the engine and vehicle and alsoconverts at least 70% of the NO to NO₂ at 400° C. (If the engine alreadyhas such a catalyst, e.g. close-coupled to the exhaust manifold, thatcatalyst would function as item 14 and items 16 and 18, to be described,could be in a separate can and retrofitted).

The gas leaving catalyst 14 passes into soot filter 16, which is of theceramic wall flow type. The NO₂ and surplus oxygen in the gas oxidisethe soot at temperatures around 250° C. with reduced accumulation ortendency to blocking. (If the engine already has a soot filter, orCRT—i.e. oxidation catalyst+soot filter—item 18, to be described, couldbe in a separate can and retrofitted).

The gas leaving filter 16 enters NOx absorber 18, which may be all inone or more distinct beds or may be at least in part present as acoating on the outlet side of filter 16. During idling operation of theengine, as when a bus is picking up passengers, NOx absorber 18substantially removes all NO₂ flowing but a limited amount of NO. The NOis odourless, however, and is less offensive to pedestrians. When,however, the engine is speeded up to running mode, as in driving to thenext bus-stop, or away from a city centre, NO is expelled, along withsome NO₂. However, movement of the bus dilutes the small amount of NO₂sufficiently to avoid offence.

EXAMPLE 1

A synthetic gas simulating diesel exhaust (except for containing noreductant) leaving soot filter 16 and having the following v/vcomposition was used:

CO₂ 10.0% H₂O 10.0% O₂  5.0% NO 60 ppm NO₂ 30 ppm N₂ balance

This gas was passed at 200 or 250° C. (as described below) over NOxabsorbents as follows:

(a) At 250° C. for 1200 seconds over a cordierite honeycomb of 400 cpsicarrying an alumina washcoat layer and an absorbent layer of BaO with Pt100 g/ft³ and Rh 10 g/f³ and minor proportions of alumina, ceria andZrO₂. The results are shown in FIG. 2;

(b) At 200° C. for 600 seconds over a cordierite honeycomb having 400cpsi, carrying an alumina washcoat and 400 g/ft³, calculated as Cu, ofCuO. The results are shown in FIG. 3;

(c) At 200° C. for 600 seconds over a cordierite honeycomb having 400cpsi, carrying an alumina washcoat, 400 g/ft³, calculated as Cu, of CuOand 100 g/f³ of Pd, calculated as metal. The results are shown in FIG.4.

For each the outlet gas was analysed for NO and NO₂ at intervals. Theresults are shown in the accompanying FIGS. 2-4. Leading data aresummarised in Table 1.

TABLE 1 Temp ° C. 250 200 200 Abst FIG. 2 FIG. 3 FIG. 4 NOx NO NO₂ NONO₂ NO NO₂ Secs 0-100 0 0  0  0  0 0  200 30 6 55 10 63 1  600 61 11 5230 71 6 1200 68 12 — — — —

From FIG. 2 it is evident that, even up to 1200 seconds (20 minutes),the emission of NO₂ is low (12 ppm), and can be expected to be below theobjectionable stench level in a real situation, such as for a bus withits engine idling while setting down and picking up passengers. At thesame time the emission level of NO steadily increases, but this causesno stench, because NO is odourless and forms NO₂ in air very slowly andat high dilution. Since the NO₂ level is rising only slowly, a yetlonger idling period would be tolerable.

From FIG. 3 it is evident that emission of NO₂ is almost zero for about100 seconds, then increases slowly to 30 ppm at 600 seconds. Thus therewould be no objectionable odour at en-route bus stops. The NO emissionafter an initial increase is about constant between 50 and 60 ppm.

From FIG. 4 the emission of NO₂ is about zero for 300 seconds andreaches only 6 ppm at 600 seconds. The NO emission increases to about 70ppm and levels out.

Evidently this absorbent provides a useful margin of safety insuppression of objectionable odour.

To test the regenerability of the absorbent, the gas composition wasadjusted, for (a) and (b) but not (c), by stopping the flow of NOx. Theinlet temperature was then ramped at about 0.25° C. per second.

Initially the measured NO₂ level did not increase much:

(a) In FIG. 2: up to 1600 seconds at 350° C. it was not over 3 ppm.Hence the initial acceleration of a bus from a bus stop would causeminor if any stench. At 2000 seconds, 470° C., desorption is rapid andthe emitted gas contains 23 ppm of NO₂ and 36 ppm of NO. The absorbentof FIG. 2 is thus to be used when the engine running mode produces veryhot exhaust gas or other heating is available;

(b) In FIG. 3: desorption equally of NO₂ and NO (10 ppm each) began veryshortly after the start of the rise in temperature. However, 100 secondslater (300° C.), preferential desorption of NO set in and at about 1000seconds the NO content (300 ppm) of the desorbed gas was about 5 timesthat of NO₂; and

(c) In FIG. 4: up to 1400 seconds at 400° C. the NO₂ level was not over6 ppm, its level at the end of the previous steady run at 200° C. Inthis time period, especially at 1000 seconds at 300° C., NO wascopiously emitted (135 ppm). This absorbent evidently takes in NO₂ butemits NO. It is especially useful when running mode exhaust is at amoderate temperature.

EXAMPLE 2

The exhaust gas from a production VOLVO 12-litre diesel engine was fedto a ceramic honeycomb-supported platinum/alumina oxidation catalyst, inwhich its content of NO was about 90% oxidised to NO₂. The resulting gaswas fed to a NOx absorber as used in the run reported in Example 1. Theresults are shown graphically in FIG. 2. The engine was operated inidling and high-speed conditions, alternating at 850 second intervals.Representative values of temperature, composition v/v and flow rate ofthe gas leaving the absorber in idling and high-speed conditions, asobtained reproducibly over many cycles, are set out in Table 2.

TABLE 2 Idle High Speed (absorbing) (regenerating) Temp ° C. 150 380NO₂, ppm 10 260 NO, ppm 180 455 O₂, % 19 10.9 CO, % 0.4 0.8 CO₂, % 1.387.14 HC, ppm 19 5.7 Total flow kg/h 259 560 NO₂, g/h 2.59 145.60 NO, g/h46.62 254.80 NO₂/NOx % 5.3 35.6

It is evident that the rate of emission of NO₂ in idling is low and maycause little if any offence. At the higher temperature the gas containsboth the NO₂ leaving the oxidation catalyst and the NO₂ desorbed fromthe absorber. However, the major part of the total NOx is NO, which isrelatively inoffensive; reformation of NO₂ by reaction of NO withatmospheric oxygen is a slow reaction, especially at the high dilutiondue to vehicle movement

The above data relate to the case in which maximal NO₂ is fed to theabsorber. Applying these results to a process in which the gas leavingthe oxidation catalyst passes through a soot filter, where the reactionNO₂+C→NO+COtakes place, it is evident that the NO₂/NOx ratio of the gas emittedwould be still lower.

1. An exhaust system for a diesel engine, which engine being operable ina plurality of modes including an idling mode during which the engineemits a relatively cool exhaust gas and a running mode during which theengine emits a relatively hot exhaust gas, the exhaust system comprisinga solid NOx absorbent, wherein NOx in an exhaust gas is absorbed by theNOx absorbent during said idling mode and is desorbed and passed toatmosphere during said running mode, during acceleration from idling torunning or during said running mode and during acceleration from idlingto running mode, as the temperature of the exhaust gas increases,wherein during all modes the engine runs lean and the exhaust gascomposition is lambda>1; and a soot filter upstream of the NOxabsorbent.
 2. An exhaust system according to claim 1, further comprisingan oxidation catalyst upstream of the NOx absorbent.
 3. An exhaustsystem according to claim 1, further comprising an oxidation catalystand a downstream filter upstream of the NOx absorbent.
 4. An exhaustsystem according to claim 1, wherein the NOx absorbent comprises atleast one of: (a) an alkali metal compound, an alkaline earth metalcompound, a rare earth metal compound and a transition metal compoundcapable of forming a nitrate, capable of forming a nitrite or capable offorming both a nitrate and a nitrite, which nitrate or nitrite havingadequate stability in absorbing conditions and being capable of evolvingnitrogen oxides, nitrogen or nitrogen oxides and nitrogen inregenerating conditions; (b) an adsorptive material; and (c) mixtures of(a) and (b).
 5. An exhaust system according to claim 4, wherein theabsorbent is catalyzed.
 6. An exhaust system according to claim 5,wherein the catalyst of the absorbent comprises at least one platinumgroup metal.
 7. An exhaust system according to claim 4, wherein the NOxabsorbent comprises (a) and the at least one alkali metal compound,alkaline earth metal compound, rare earth metal compound or transitionmetal compound is an oxide thereof.
 8. An exhaust system according toclaim 4, wherein the NOx absorbent comprises (b) and the at least oneadsorptive material is selected from the group consisting of zeolites,carbons and high surface-area oxides.
 9. A diesel engine including anexhaust system according to claim
 1. 10. A heavy-duty diesel engineaccording to claim
 9. 11. An exhaust system according to claim 1,wherein the NOx absorbent comprises BaO.
 12. An exhaust system accordingto claim 1, wherein the NOx absorbent comprises CuO.